1. Field of the Invention
The present invention relates to the recovery of metals from aqueous solutions, and more particularly to the field of recovery of base and precious metals from ground water, surface water, domestic and public water, from mines, industrial metals plants discharge and plating industry.
2. Statement of the Problem
Metals are typically slightly soluble in water. In many instances it is desirable to remove metal from aqueous solutions. For example, in producing drinking water, it is desirable to remove metals because of the health risks associated with certain metals. Removing certain metals from industrial waste before disposing into the environment may be required by regulation. Obviously, where significant volumes of aqueous solutions containing precious metals, such as gold or silver, are available, it is desirable to remove these metals for their intrinsic value. Gold is increasingly being used in the semiconductor industry. As aqueous wash solutions replace harsher, fluorine-based wash solutions, the semiconductor industry may become a source of significant volumes of gold-bearing aqueous solutions. Groundwater in regions containing gold-bearing ores often contain small concentrations of gold. One of the greatest sources of large volumes of water containing small concentrations of metals, and particularly precious metals, is mines.
Throughout the world there are many subterranean gold and silver mines that were once prosperous but have been closed or abandoned because their yields ceased to justify large scale commercial mining. Nevertheless, quantities of gold, silver, and other heavy metals remain in the ores surrounding the mines, although perhaps not in sufficient concentrations to warrant commercial mining. When subterranean mines are closed or abandoned, ground water often collects in the mine shafts. Water, as it flows from the surface into the earth through the various rocks surrounding the mine, dissolves metals from the ores. There are many abandoned mine shafts containing significant quantities of water with dissolved metals in solution although the concentrations are low, often 50 parts per million ("ppm")or less. However, no commercially available process exists to economically remove the dissolved metals from the water.
There are many known processes for removing metals from an aqueous solution. However, these known processes become very inefficient and cease to be cost effective at such low concentrations.
One such process, electrowinning, has long been used in conjunction with heap leach-carbon adsorption methods for gold and silver mining. In the context of the recovery of gold (although this process can be varied to recover both gold and silver), a low grade ore is formed into a heap. A dilute alkaline cyanide solution (although other solutions are also used) is sprayed on the heap and allowed to flow through the heap to a recovery point. The alkaline cyanide solution recovered contains molecular gold, and this solution is referred to as the "pregnant solution." One highly favored method of recovering gold from the pregnant solution is carbon stripping. In carbon stripping, the pregnant solution is passed through activated carbon, which adsorbs the gold. Various methods are available for "desorption" of gold from the activated carbon. A well known method is to leach the carbon with an aqueous solution of sodium hydroxide and sodium cyanide to remove the gold from the activated carbon. This stripping solution is then passed through one or more electrolytic cells in the process known as "electrowinning."
Electrowinning is a direct current (DC) electrolytic process involving low voltage and high current. An electrowinning cell consists of an anode, a cathode, and a corrosion-resistant container. As current is passed through a cell, gold and other metals are removed from the solution and deposited at the cathode. One popular cell design, the "Zandra" cell, consists of three containers nested inside one another. (Other cell designs are know; See, e.g., David A. Milligan, et al., Introduction to Evaluation, Design, and Operation of Heap Leaching Projects, 137-151 (Dirk van Zyl ed. 1988)). The inner container is perforated and serves as the cathode compartment in which is placed a feed tube and a quantity of stainless steel wool, which, being grounded, functions as a cathode. The second outer steel container surrounding the cathode is the anode. The third outer container functions as an overflow container. The metal-bearing stripping solution is introduced into the first container through the feed tube, and flows up through the steel wool cathode to overflow the first container into the second, and then overflows the second into the third where it is then removed and recirculated. (See, e.g., J. B. Zandra, et al., Process for Recovering Gold and Silver From Activated Carbon by Leaching and Electrolysis, U.S. Bureau of Mines Report of Investigations 4843 (1952)). The stripping solution typically has a concentration of 50 to 2,000 ppm of gold, although it may assay as high as 3500 p.m. However, the efficiency decreases dramatically with a decrease in concentration, with 100% efficiency at 2000 ppm to zero efficiency at 1 ppm (See Milligan at 145). The power required for electrowinning is determined by cell voltage and current. Typically, 2-3 volts are used, with current depending on the size of the cell.
In a typical commercial operation (as set forth in D. M. Duncan, T. J. Smith, How Cortez Gold Mines Heap-Leached Low Grade Ores at Two Nevada Properties, E/MJ July 1977) at a flow rate of 13 GPM of a stripping solution, an electrowinning cell containing nine cathodes consumed 190 amps at 2.5 volts. Where the feed solution had a concentration of 0.7 oz. gold/ton (24 ppm), the discharge from the cell contained 0.07 oz. gold/ton (2.4 ppm). Hence, 475 watts were consumed to treat 13 GPM to recover 21.6 ppm., or 90% of the gold in the solution. Besides the cost of electricity, this process obviously includes other high overhead and operational cost, such as the cost of forming the heap, washing the heap with the leaching solution, pumping the leaching solution through a carbon stack, and washing the carbon with the stripping solution.
It should be kept in mind that the above data are for a process using a caustic solution, which has a much higher conductivity than groundwater containing low concentrations of metals. When there are no electrolytes present except for dissolved metallic ions, the resistance of the aqueous solution is much greater, so higher voltages are required to obtain the high current flow on which the electrowinning process depends. Since the groundwater in mines is essentially pure, it would be undesirable to add harsh, environmentally dangerous electrolytes (such as sodium hydroxide) to enhance conductivity as doing so would only create a waste disposal problem that would further add to the cost of recovery. The above example from the Cortez mine is illustrative of the electrical energy consumption where a highly conductive electrolyte is present. However, groundwater from mines typically does not have a high conductivity, as the harsh electrolytes used in the Cortez mining operation are not present. When the electrowinning process is used with substantially pure water containing low concentrations of metals, the voltage required to attain the current flow required by electrowinning to accomplish plating increases dramatically, with as much as 150 or even 300 volts required. Hence, power consumption is also dramatically increased (with 150 volts and 25 amps constituting 3,750 watts). Furthermore (as stated by Milligan at 145), the efficiency of the electrowinning process approaches zero at concentrations of gold less than 1 ppm.
When considering the low efficiency at low concentrations, the high power consumption (both at the electrodes and in converting AC voltage to DC voltage), and the cost of electrical power, the electrowinning process is not economically feasible where concentrations of gold and the conductivity of the solution are low. Thus, what is needed is an apparatus and a process to remove metals, including precious metals such as gold, from water having low concentrations (such as 50 ppm to 0.008 ppm) that are highly efficient at such low concentrations, removing the majority of gold while consuming little electrical power and also having low capital and operating costs.
3. Solution to the Problem
The present invention provides a novel solution to the aforementioned problems by providing a cost effective system and process for removing precious metals (gold and silver), and other base metals from aqueous solutions having low concentrations of these metals. The present invention consumes little electrical power to treat a relatively high flow of water containing as little as 2 parts per billion for gold yet removing 97% of the gold and removing 90% to 100% of such metals as iron, aluminum, lead, arsenic, cadmium, zinc, etc. This invention can be used with either direct or alternating current, although alternating current is preferred since conversion of readily available alternating current to direct current would involve a needless expense and energy loss. This invention does not require the use or addition of any harsh chemical additives to increase the conductivity of the water since it does not rely on high current flow. Since electrolytic additives are not required, when treating groundwater from mines, the water can often be deposited without any further treatment directly into the environment. Furthermore, the apparatus of this invention can be inexpensively built using components available at most local hardware stores, and can be assembled on the back of a small trailer, allowing it to be easily transported between remote locations.
The present invention includes, in one embodiment, a system having seven cylindrical electrodes, each connected to a high voltage power source (preferably 10,000 VAC). Each electrode is enclosed in a grounded stainless steel tube inside a plastic tube through which metal bearing water is passed, and the tubes are connected end-to-end. Each electrode is electrically insulated and sealed from the water to minimize the electrical current flowing through the system. After flowing past the electrodes and the ground, the water flows through at least one collector (two in the preferred embodiment), each having a quantity in one manganese dioxide and in the other carbon. Molecular, micron, ionic, as well as colloidal gold and other metals are deposited onto these filters after passing through the electrical field generated between the electrode and ground. Particles larger than colloidal gold may also be filtered out by the filter tanks or may settle out in a settling tank (an optional accessory to the present invention) after the water passes through the collectors. Typically, in a system using seven such electrodes in series and two collectors, the current flow is approximately 23 milliamps. The filters are granulated iron-grabbing resin, granulated carbon, and as a final filter at least minus 30 to 70 mesh clean quartz sand.